Web materials comprising heat fusible material, either fibrous webs, films or laminates thereof, are used in a variety of applications, such as protective clothing and hygiene articles, especially disposable absorbent personal care articles, like diapers, sanitary napkins and incontinence guards. In most of such applications the web materials must be joined, which is conventionally done by ultrasonic welding, heat bonding, laser welding or by adhesive. Bonding by ultrasonic welding, heat bonding or laser welding is usually done by forming a bonding pattern comprising a plurality of bonding sites in which the heat fusible components of the web materials are fused and thus bonded together.
The bonding is done in a process line, in which material variations as well as process variations may occur, which can effect the exact positioning of the bonding pattern with respect to the overlapping side edges of the web materials. If the bonding pattern ends up being located a bit offset with respect to the overlap, an edge of the overlap may protrude from the surface of the joined web materials. This will reduce the quality of the seam and of the article and if used in an article worn against the skin of a wearer such a protruding edge may cause skin irritations.
U.S. Pat. No. 5,919,539 discloses ultrasonic seaming of spunbonded polyolefin sheets for example used in protective clothing. The overlapping sheet edges are fed between an ultrasonic horn and a patterned roll that has a raised pattern corresponding to the desired bonding pattern. A waffle-like bonding pattern is created and the edges of the sheet are welded over so as to eliminate protruding sheet edges along the seam.
U.S. Pat. No. 5,626,574 refers to a disposable diaper having a front and a back body panel bonded to each other by ultrasonic welding to form side seams. The welded side seams are defined by patterns of intermittently arranged bonding sites of different shape. The purpose is to create a welding pattern which allows the side seams to be torn open longitudinally along the welded zones.
U.S. Pat. No. 4,938,817 shows a welding pattern for joining two overlapping material webs, said welding pattern comprises short dashed lines extending in parallel to the edges of the fabric.
U.S. Pat. No. 4,205,679 discloses a disposable undergarment in which a strong ultrasonic weld seam is formed by a plurality of weld lines extending in longitudinal direction of the overlapping side edges spaced inwardly from the edges of the overlap.
There is a need for an improved bonding pattern for seams joining together at least two web materials of the above mentioned kind, said bonding pattern providing both a strong seam along the overlap and a reduced risk for protruding edges along the seam